It has been demonstrated that the incorporation of nano-sized structures into macroscopically-sized products can improve product characteristics such as electrical characteristics, mechanical characteristics, thermal characteristics, and optical characteristics, just to name a few. However, the development of commercially available products that successfully incorporate nano-sized structures has often been stymied due to problems associated with formation of the nanostructures. For example, methods for forming bulk quantities of nanostructures have proven difficult to develop, especially when the costs associated with scale-up of existing laboratory-sized formation methods are considered. Moreover, methods for forming nanostructures in bulk such that the individual structures as formed exhibit little variation one to another in size and shape has also proven problematic.
Solid-state energy conversion, and in particular solid-state thermoelectric (TE) energy conversion, is just one exemplary technology area in which improved methods for forming high quality nanostructures in bulk could lead to improved product development. Thermoelectric solid-state energy conversion materials can be beneficially utilized in products to provide localized, compact energy conversion. These materials are becoming more common in a variety of applications, including cooling of electrical components (computer drives, laser diodes, etc.), localized climate control (vehicle seat warmers/coolers, food and beverage heaters/coolers, etc.), and in consumer products such as watches, lamps, and the like.
State of the art thermoelectric materials usually exhibit a dimensionless figure of merit (ZT) on the order of unity (ZT≈1). An enhancement of ZT by a factor of 2 or greater could provide improved thermoelectric materials for use in refrigeration, heating, and power generation applications. It has been predicted that nanoscaled thermoelectric materials might exhibit superior properties to those of their micro- or macroscaled counterparts. Following this prediction, a significant increase in ZT (i.e., ZT>2) was reported for nanoscaled systems constituently based on commonly used large scale TE materials (see, for example, R. Venkatasubramanian, et al., Nature, 413, 597 (2001), and T. C. Harman, et al., Science, 297, 2229 (2002)). In fact, it was known in the 1980s that the ZT for highly disordered alloys of PbTe with a mean grain size of 1 μm could be roughly 10% higher than the equivalent but single-crystal value.
Accordingly, one promising route to preparing thermoelectric materials with enhanced ZT may be to incorporate nanostructures formed of thermoelectric materials into a bulk phase matrix, in expectation that the phonon scattering at grain boundaries could significantly reduce the lattice thermal conductivity while the electrical properties could be largely preserved. However, in order to economically provide such enhanced thermoelectric materials, improved methods for forming the thermoelectric nanostructures is necessary. In particular, methods must be developed for forming such nanostructures economically in bulk. Moreover, such methods would ideally also provide the nanostructures with relatively little variance in size and with good crystalline nature.
Improved methods for forming nanostructures in bulk and within a narrow size distribution range can benefit other technologies as well, in addition to thermoelectrics. In fact, the ability to provide nanostructures in bulk and with a predetermined particle size can provide a new control parameter in formation of such composite materials that can be beneficially utilized to control and/or improve bulk characteristics such as strength characteristics, electrical characteristics, and the like for many different applications.